Spacecraft are subject to environmental extremes, including vacuum, charged particle flux, and temperature extremes; high temperatures attributable to solar radiation when insolated, and low temperatures due to radiation into space when in eclipse. These environmental conditions are met, in part, by the use of thermal control blankets or coverings. Important aspects of such coverings include low weight, low cost, and consistency and ease of application and reapplication. Some thermal control coverings are transparent to radio-frequency (RF) electromagnetic waves, as described, for example, in U.S. Pat. No. 5,283,592, issued Feb. 1, 1994 in the name of Bogorad et al., and are intended for use in areas of the spacecraft containing antennas. Other thermal coverings, in which RF transparency is not important, are intended for use on other portions of the spacecraft. In general, non-RF-transparent coverings have characteristics which prevent solar radiation from reaching the spacecraft itself, either by reflecting the insolation, absorbing it, or both. In effect, such coverings are opaque to insolation, as seen from the protected side (the spacecraft side). Also, such coverings generally have a conductive outer surface, so that the flux of charged particles does not result in a buildup of charge on the outer surface of the covering, which might result in an electrical arc, and possible damage to the spacecraft.
One thermal covering or blanket for covering portions of spacecraft consists of essentially opaque black polyimide (KAPTON) sheet, which is rendered electrically conductive by carbon loading. Another prior-art thermal covering or blanket consists of natural (transparent) polyimide sheet, with a coating of aluminum or other appropriate metal applied to the inside of the sheet to reflect insolation and thereby render the covering opaque, and with a thin, transparent layer of indium-tin oxide (ITO) on the exterior surface for electrical conductivity. The thermal control sheets, when applied to the spacecraft, must have their electrically conductive portions electrically coupled to the spacecraft body, in order to distribute charge in a manner which tends to reduce the possibility of arcing. In the prior art, this has been accomplished by metal tabs, which may be made from aluminum, which are connected between the outer surfaces of the thermal blankets or coverings, and between the outer surfaces of at least some of the blankets and a conductive portion of the spacecraft body.
The metal tabs have been found to have coefficients of expansion which differ from that of the polyimide sheets, so that there is a potential for tearing of the sheets or loosening of the electrical connection under the influence of repeated cycles of insolation. This is solved by protecting the metal tabs from insolation by means of an additional sheet of polyimide material similar to the thermal covering material. Improved thermal coverings are desired for spacecraft.